This invention relates to electrophotography and more particularly, to an improved method of preparing an electrophotographic imaging member.
Generally, electrophotographic imaging processes involve the formation and development of electrostatic latent images on the imaging surface of a photoconductive member. The photoconductive member is usually imaged by uniformly electrostatically charging the imaging surface in the dark and exposing the member to a pattern of activating electromagnetic radiation such as light, to selectively dissipate the charge in the illuminated areas of the member to form an electrostatic latent image on the imaging surface. The electrostatic latent image is then developed with a developer composition containing toner particles which are attracted to the photoconductive member in image configuration. The resulting toner image is often transferred to a suitable receiving member such as paper. The photoconductive members include single or multiple layered devices comprising homogeneous or heterogeneous inorganic or organic compositions and the like. One example of a single layer photoconductive member containing a heterogeneous composition is described in U.S. Pat. No. 3,121,006 wherein finely divided particles of a photoconductive inorganic compound is dispersed in an electrically insulating organic resin binder. The commercial embodiment usually comprises a paper backing containing a coating thereon of a binder layer comprising particles of zinc oxide uniformly dispersed therein. Useful binder materials disclosed therein include those which are incapable of transporting for any significant distance injected charge carriers generated by the photoconductive particles. Thus, the photoconductive particles must be in substantially contiguous particle to particle contact throughout the layer for the purpose of permitting charge dissipation required for cyclic operation. Generally, about 50 percent by volume of photoconductive particles is usually necessary in order to obtain sufficient photoconductive particle to particle contact for rapid discharge. Other known photoconductive compositions include amorphous selenium, halogen doped amorphous selenium, amorphous selenium alloys including selenium arsenic, selenium tellurium, selenium arsenic antimony, halogen doped selenium alloys, cadmium sulfide and the like. These inorganic photoconductive materials are usually deposited as a relatively homogeneous layer on suitable conductive substrates. Some of these inorganic layers tend to crystallize when exposed to certain vapors that may occasionally be found in the ambient atmosphere. Moreover, the surfaces of selenium type photoreceptors are highly susceptible to scratches which print out in final copies. Layered photoreceptors, whereby the photogeneration function and the charge transport function are performed by separate layers, are well known, as disclosed, for example in U.S. Pat. No. 3,041,166 to J. Bardeen. Recently, there has been disclosed layered photoresponsive devices comprising charge transport layers comprising photogenerating particles and charge transport layers deposited on conductive substrates as described, for example, in U.S. Pat. No. 4,265,990 and overcoated photoresponsive materials containing a hole injecting layer, a hole transport layer, a photogenerating layer and a top coating of an insulating organic resin, as described, for example, in U.S. Pat. No. 4,251,612. Examples of photogenerating layers disclosed in these patents include trigonal selenium and various phthalocyanines and hole transport layers containing certain diamines dispersed in inactive polymer resin materials. The disclosures of each of these patents, namely, U.S. Pat Nos. 4,265,990 and 4,251,612 are incorporated herein by reference in their entirety. Other representative patents containing layered photoresponsive devices include U.S. Pat. Nos. 3,041,116; 4,115,116; 4,047,949 and 4,081,274. These patents relate to systems that require negative charging for hole transporting layers when the photogenerating layer is beneath the transport layer. Photogenerating layers overlying hole transport layers require positive charging but must be less than about 2 micrometers for adequate sensitivity. While the above described electrophotographic imaging members may be suitable for their intended purposes, there continues to be a need for improved devices. Thus, in summary, layered photoreceptors, whereby the photogeneration function and the charge transport function are performed by separate layers, are well known and many such structures are used in commercial xerographic copiers and printers. These layers can be made from inorganic materials, for example, chalcogenides; organic materials, for example, polymers with electronically active additives, and combinations of organic and inorganic materials. The charge generator layer typically consists of a polymer binder to which is added an organic or inorganic photoactive pigment. For lower loadings of pigment particles in a charge generating binder layer, the coatings were necessarily thick in order to secure sufficient optical absorption during imagewise exposure. Unfortunately, with thicker charge generation layers, light absorption by separate photogenerating pigment particles resulted in space charge build up and high internal fields that eventually led to dark decay and instability with electrical charge-erase cycling. In the rare case where the binder of the charge generating particles is ambipolar, a photoreceptor that was charged negatively would permit negative charges forming on the particles to travel to the conductive ground plane thereby avoiding space charge build up. However, for most film forming binders, the concentration of photoconductive pigment particles should be sufficiently high to afford particle-to-particle contact so that the negative charges are provided with paths to travel to the ground plane. The layers should also be thin to minimize the distance the positive and negative charge must travel in the generator layer. Unfortunately, high concentrations of pigment particles in a binder matrix is difficult to achieve. A common technique for preparing charge generation layers is to first place particles in a solution containing dissolved film forming binder material. Generally, with these mixtures, it is difficult to obtain charge generating binder layers containing high levels of loadings of pigment particles in the 70 percent to 80 percent by volume range. The coating of the charge generation pigment in a binder also present other problems. These include incompatibility of the charge generation transport layer polymer binders and/or solvents with the charge transport layer polymer binders and/or solvents. Also, it is difficult to form uniform, submicron, generator layers having high concentration of pigment particles from mixtures of charge generation pigment in a binder dissolved in a solvent. Moreover, swelling of the bottom layer can occur when coated with a second layer. Photogenerating layers have also been prepared by dissolving squarylium compounds in a solvent and thereafter applying the resulting solution to a substrate. This approach limits the range of materials that may be utilized in the photogenerating layer. Moreover, the deposited coating often does not adhere well to the underlying substrate and/or to subsequently applied layers. Some organic charge generating materials such as phthalocyanines are coated by vacuum deposition. Vacuum deposition, however, requires expensive and complex equipment and may result in poor adhesion between the evaporated layer and the solvent coated layer.